Combined biochar and metal-immobilizing bacteria reduces edible tissue metal uptake in vegetables by increasing amorphous Fe oxides and abundance of Fe- and Mn-oxidising Leptothrix species

In this study, a highly effective combined biochar and metal-immobilizing bacteria (Bacillus megaterium H3 and Serratia liquefaciens CL-1) (BHC) was characterized for its effects on solution Pb and Cd immobilization and edible tissue biomass and Pb and Cd accumulation in Chinese cabbages and radishes and the mechanisms involved in metal-polluted soils. In the metal-containing solution treated with BHC, the Pb and Cd concentrations decreased, while the pH and cell numbers of strains H3 and CL-1 increased over time. BHC significantly increased the edible tissue dry weight by 17–34% and reduced the edible tissue Pb (0.32–0.46 mg kg−1) and Cd (0.16 mg kg−1) contents of the vegetables by 24–45%. In the vegetable rhizosphere soils, BHC significantly decreased the acid-soluble Pb (1.81–2.21 mg kg−1) and Cd (0.40–0.48 mg kg−1) contents by 26–47% and increased the reducible Pb (18.2–18.8 mg kg−1) and Cd (0.38–0.39 mg kg−1) contents by 10–111%; while BHC also significantly increased the pH, urease activity by 115–169%, amorphous Fe oxides content by 12–19%, and relative abundance of gene copy numbers of Fe- and Mn-oxidising Leptothrix species by 28–73% compared with the controls. These results suggested that BHC decreased edible tissue metal uptake of the vegetables by increasing pH, urease activity, amorphous Fe oxides, and Leptothrix species abundance in polluted soil. These results may provide an effective and eco-friendly way for metal remediation and reducing metal uptake in vegetables by using combined biochar and metal-immobilizing bacteria in polluted soils.